What is the idiomatic Haskell solution for dependency injection?

E.g., suppose you have an interface frobby, and you needed to pass an instance conforming to frobby around (there might be multiple varieties of these instances, say, foo, and bar).

Typical operations would be:

  • functions that take some value X and return some value Y. E.g., this might be a database accessor, taking a SQL query & a connector and returning a dataset. You might need to implement postgres, mysql, and a mock test system.

  • functions that take some value Z and return a closure relating to Z, specialized to a given foo or bar style, chosen at runtime.

One person solved the problem as follows:

http://mikehadlow.blogspot.com/2011/05/dependency-injection-haskell-style.html

But I don't know if that's the canonical way to manage this task.

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